1. Technical Field
This document relates to a center tube configuration for a multiple spiral wound forward osmosis element.
2. Background
A common design for membrane elements in industrial scale systems uses flat sheet membrane rolled into spiral elements. This membrane element design has a series of membrane envelopes which are sealed to, and communicate with, a perforated central core tube. Spacer nets are inserted into the envelopes and between the envelopes, then the envelopes are wrapped tightly around the central core. The elements are then loaded end-to-end in long housings and the water to be filtered is pumped axially down the housing so that it flows through the spacer nets on the outside of the envelopes. Applied pressure then forces water through the membrane. The nets inside the membrane envelopes allow the filtered water to pass to the center tubes which convey water out of the housing.
Interest has gained in large scale forward osmosis (FO) applications such as seawater desalination, municipal wastewater reduction, production of fertilizer from methane bioreactor waste, and the like. All of these applications require industrial scale systems with thousands of square meters of FO membrane.
Unlike pressure driven filtration, FO requires a second fluid, or osmotic agent (OA) or brine, to pull water across the membrane. In a spiral wound FO design, OA must be brought to flow through the entire inside of the membrane envelopes, while the fluid to be concentrated (i.e., feed fluid or solution) flows through the spacers on the outside of the envelopes. Osmosis causes the feed fluid to be concentrated while consequently diluting the OA. Thus, in a spiral wound FO system there is a feed solution that provides the contaminated water source for purification and a draw solution that provides an osmotic agent, such as syrup or brine.
Such FO spiral wound systems can be used for single elements with at most 5 psi pressure drops. However, there are significant problems associated with scaling up such single FO spiral wound systems because if the elements are connected serially, the pressure drops are additive and reduce the utility of such systems as higher capacity water purification systems.